Pep Guardiola has spent more time thinking about how to beat Jurgen Klopp's Liverpool than any other opposition in his career.
Manchester City and Liverpool have been comfortably the best two teams in the Premier League over the last two years, with the Blues becoming the first team in a decade to retain the title in 2019 before being blown away by their Merseyside rivals this season.
There may be friction between the clubs away from the pitch - there was surprise at the Etihad that Liverpool tried to involve themselves in the Champions League hearing at the Court of Arbitration for Sport given they had already qualified for next season's competition - but on it there is immense respect between the two coaches.
Of the many battles that Guardiola has had during a coaching career spanning more than a decade, he views Klopp's Liverpool as the toughest.
"The toughest opponent I've ever faced in my career is this Liverpool from last year and this one," Guardiola told DAZN.
"If you let yourself be dominated, they lock you in the area and you can't escape. When you dominate them, they run into space like no one else. They are very fast going backwards. They are very strong strategically. Their players have great mental strength. [Klopp] is the rival who has made me think about how to beat him the most."
Guardiola's rivalry with Klopp has largely been done on the grass rather than in press conferences, making it less explosive than previous challenges with Jose Mourinho at Real Madrid.
City's coach also believes that the lower profile of Liverpool when he took over at the Etihad in 2016 has made them a tougher rival because of the dramatic improvement they have shown to match and then move past the Blues.
"I have always said that Real Madrid are a very strong team in my career, they helped me to become a better coach with tough matches and competitions with Jose Mourinho, [Manuel] Pellegrini and all the coaches they had," Guardiola added.
"If you ask me which has been the most difficult rival for me to face, it has been Liverpool. The first years when I arrived in England, Liverpool were a bit weaker than now and Real Madrid were stronger. Now, this Liverpool are the hardest I have come across in my coaching career."
